Progressive Retinal Atrophy (PRA) is a heterogeneous class of retinal disorders that share a broadly similar clinical disease phenotype, and affect the dog (Canis familiaris) (Aguirre, 1976). The clinical features include: initial night blindness followed by reduction in photopic vision leading to complete blindness; reduction in retinal vessels, and retinal thinning; abnormalities in an electroretinogram (“ERG”); and the development of cataracts. Diseases of this group are typically inherited by means of an autosomal recessive gene defect although dominant and X-linked forms of PRA also are recognized (Kijas et al., 2002; Zhang et al., 2002). PRA may be classified into developmental and degenerative diseases. The developmental class comprises several genetically distinct diseases expressed cytologically in the immediate postnatal period when visual cells in the canine retina begin to differentiate (Acland et al. 1989). In contrast, the degenerative class represents defects in which photoreceptor cells degenerate after having differentiated normally—this class includes the specific disease termed progressive rod-cone degeneration (prcd). This specific form of PRA is an autosomal recessively inherited, late-onset retinal degenerations affecting several different breeds of dog (Aguirre and Acland, 1988).
Mutations at the prcd ‘gene locus account for all of the autosomal recessive late-onset hereditary retinal degenerations recognized to date in dogs. By cross-breeding experiments, it has been determined that the prcd gene locus is responsible for progressive retinal atrophy in poodles (toy, and miniature), cocker spaniels (American, and English), Labrador retrievers, and Portuguese water dogs (see, e.g., Aguirre and Acland, 1988, Aguirre and Acland, 1991; Pearce-Kelling et al., 2002). Cross-breeding experiments suggest the same mutation in the F04 gene (which is gene responsible for prcd) is also present in several other breeds either in dogs affected with prcd; or carriers of the disorder. However, based on clinical and genetic parameters consistent with disease caused by mutations at the prcd gene locus, other breeds of dogs suspected of having prcd as the form of observed progressive retinal atrophy include akita, basenji, border collie, English mastiff, English springer spaniel, Havanese, lowchen, samoyed, standard wirehaired dachshund, Tibetan terriers, Bernese mountain dog, and miniature schnauzer. Depending on the breed of the dog, different mutations responsible for allelic variants of the prcd gene locus can regulate the rate of progression, but not the phenotype, of photoreceptor degeneration.
Clinical diagnosis of prcd disease is complicated by the need for sophisticated testing methods such as ERG, and by the late onset of the disease. The age at which the disease can be diagnosed by current methods may be past the dog's reproductive life. For example, in English cocker spaniels, progressive retinal atrophy may be diagnosed by ERG at three years of age, and by ophthalmoscopy at 5-8 years of age. This late age of diagnosis results in the dissemination of the undesirable trait within the population, and an increase in the disease frequency.
The estimated prevalence of progressive rod-cone degeneration differs among affected breeds. It is believed that approximately 2% of Labrador retrievers more than 2 to 3 years old are affected with progressive rod-cone degeneration; if so, then the proportion of Labrador retrievers expected to be heterozygous at the prcd locus could be as high as 24%. In poodles and cocker spaniels, the disease rate is higher than that observed in Labrador retrievers, and hence, the carrier rate would be expected to be higher. From the results of a survey of Portuguese water dogs, the calculated carrier frequency is approximately 40%.
Traditional measures for controlling inherited diseases in a population included performing “test” matings to identify carrier dogs, and to eliminate the identified carriers from breeding programs, thereby reducing the frequency of genetic disease in a breed. In a test mating, the dog being evaluated as a potential carrier of the genetic disease is mated with a dog known to be affected with the disease. Progeny are then observed for absence or presence of the disease, and a litter equal to or larger than 6, all of which are unaffected offspring, typically “clears” the dog from being a carrier. While test matings have been effectively used for breeds having large litter sizes, and for diseases which are early onset, such a procedure is not practical for reducing the frequency of prcd. In addition to the disadvantages of test matings such as great expenses in time and effort incurred to clear a dog and that affected dogs can be born if the dog to be evaluated is a carrier, test matings are not particularly suited for detection of carriers of prcd because of the late onset of clinical symptoms associated with the disease, and because some of the breeds affected have small litters (too small for establishing statistical probability).
Although the gene carrying the mutation or mutations that cause prcd has previously been unknown, genetic linkage studies in prcd families have shown that the gene that causes the disease in dogs resides on the centromeric end of canine chromosome 9, an area that is homologous to the telomeric end of the long arm of human chromosome 17 (Acland et al., 1999; Sidjanin et al., 2003).
In spite of the extensive efforts in the art to find the gene responsible for prcd, until now the gene has remained elusive. Identification, isolation, cloning, and sequencing of the prcd gene would enable the design and manufacture of products useful for the diagnosis and screening for prcd. Therefore, there has been an ongoing need in the canine breeding industry for a genetic test that permits direct identification of dogs that have the prcd form of progressive retinal atrophy (e.g., before detectable onset of clinical symptoms), as well as permitting the genotyping of dogs at risk for prcd to establish if they are affected, carriers or genetically normal.